probinglocalenvironmentsbytime-resolvedstimulated … · 2011. 7. 31. · anarei,1...

Hindawi Publishing CorporationInternational Journal of SpectroscopyVolume 2012, Article ID 271435, 5 pagesdoi:10.1155/2012/271435

Research Article

Probing Local Environments by Time-Resolved StimulatedEmission Spectroscopy

Ana Rei,1 Graham Hungerford,2 Michael Belsley,1 M. Isabel C. Ferreira,1

and Peter Schellenberg1

1 Centro de Fısica, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal2 HORIBA Jobin Yvon IBH Ltd., Skypark 5, 45 Finnieston Street, Glasgow G3 8JU, UK

Correspondence should be addressed to Peter Schellenberg, [email protected]

Received 31 July 2011; Revised 22 September 2011; Accepted 22 September 2011

Academic Editor: A. M. Brouwer

Copyright © 2012 Ana Rei et al. This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Time-resolved stimulated emission spectroscopy was employed to probe the local environment of DASPMI (4-(4-(dimethylam-ino)styryl)-N-methyl-pyridinium iodide) in binary solvents of different viscosity and in a sol-gel matrix. DASPMI is one of themolecules of choice to probe local environments, and the dependence of its fluorescence emission decay on viscosity has beenpreviously used for this purpose in biological samples, solid matrices as well as in solution. The results presented in this papershow that time-resolved stimulated emission of DASPMI is a suitable means to probe the viscosity of local environments. Havingthe advantage of a higher time resolution, stimulated emission can provide information that is complementary to that obtainedfrom fluorescence decay measurements, making it feasible to probe systems with lower viscosity.

1. Introduction

Fluorescent stilbenoid dyes are among the classes of chro-mophores most widely used in staining biological samples tobe investigated by fluorescence microscopy methods [1–4].Stilbenoid dyes show complex excited state dynamics, suchas isomerisation processes, which are often accompaniedby the emergence of charge transfer states [2, 5–9]. Animportant consequence of these dynamics is that theirfluorescence properties are particularly sensitive to the localenvironment, which not only increases the contrast betweensample features, but also delivers specific information onlocal properties. In the particular case of DASPMI (4-(4-(dimethylamino)styryl)-N-methyl-pyridinium iodide), sol-vent polarity as well as viscosity can influence the fluores-cence properties of this dye. To a good approximation, thepeak of the emission spectrum can be used as an indicatorfor the polarity of its surroundings, while the fluorescencelifetime varies with the viscosity of the local environment[8, 9].

It has been previously demonstrated that the fast molec-ular dynamics processes in DASPMI, such as the rearrange-ments of the dye upon transitions to intramolecular charge

transfer states, are sensitive to the local viscosity [2, 10,11]. In particular, the fluorescence lifetime of DASPMI isquite sensitive to the local viscosity, making fluorescencelifetime measurements a method of choice to probe themicroheterogeneous environments in confining media suchas sol-gel based or biological systems [10, 12, 13]. Previousstudies have shown that when DASPMI is incorporatedin sol-gel-derived media, the lifetime of the fastest decaycomponent can increase up to a hundred fold while thelifetime of the longer decay components tend to stabilize[13].

Sol-gel-derived media are promising materials for appli-cations as optical biosensors [14–16]. Their porosity, robust-ness, transparency in the near UV and visible part of thespectrum along with the inherent flexibility of the sol-gelmethod have allowed the encapsulation of a wide range ofbiomolecules, including proteins, DNA, and even whole cells[17, 18], and their structure allows for a limited substancemobility within the amorphous porous network. Otherdirections of research are motivated by the requirement toproduce new materials to be used in electronics, communi-cation, energy, and other high-technology fields [19, 20]. Theencapsulation of enzymes is particularly attractive because of

2 International Journal of Spectroscopy

the efficiency, specificity, and selectivity of these biocatalysts.There are many publications on the successful entrapmentof enzymes and for a review see [21]; however, some majorissues still remain, explaining why this is still an intense fieldof investigation. Knowledge about the microenvironmentcomposition inside the pores and about the molecularmobility of the enzyme and substrates/products of reactionis crucial to comprehend the enzyme kinetics in confiningmedia. In this frame, measurements concerning the viscosityevolution in a sol-gel sample would be important forcharacterizing mass transport inside the matrix [14, 22].

In the present work, we report on complementary mea-surements employing femtosecond time-resolved stimulatedemission to elucidate the DASPMI kinetics in homogeneoussolution and a microheterogeneous sol-gel medium. As theexperimental time resolution in these techniques is of order100 fs, the technique could be employed to probe faster decayprocesses compared to fluorescence lifetime measurements.This would allow one to explore the faster kinetics affiliatedwith lower viscosity matrices, extending the range of thetechnique.

In this work, the principal applicability of the methodis tested. Apart from measurements on matrices producedby the sol-gel path, we probe glycerol-water mixtures ofvarious viscosities. As was the case for the fluorescence decaysignals using fluorescence upconversion experiments [23,24], we are able to identify wavelength-dependent kinetics.With the increased time resolution of an ultrafast lasersystem, we can clearly identify dynamic processes in DASPMIon a subpicosecond scale.

2. Experimental Setup

The pump-probe measurements were carried out using theoutput of a Coherent Legend Ti : sapphire-chirped pulseregenerative amplifier system seeded by a Coherent MiraTi : sapphire oscillator which in turn was pumped by a 5 Wfrequency-doubled Nd-laser. The system generated a 1 kHzpulse train at a wavelength of 800 nm with a FWHM ofaround 120 fs and pulse energies of 2 mJ. A small portion ofthese amplified pulses were used to generate the excitationpulses centred at 400 nm by frequency doubling in a BBOcrystal. The majority of the pulse intensity is used to generatethe probe beam by pumping an ultrafast optical parametricamplifier (OPA) from Light Conversion (TOPAS). The timedelay Δt between the probe and the excitation beam wasscanned with a delay line (Standa, Lithuania 8MT167-100)with minimum step sizes corresponding to a delay of around4 fs. The signals were collected with a fast PIN-biased photo-diode (Thorlabs DET10A/M). A modular boxcar integratorsystem SR-240/SR-245/SR-250/SR-280 (Stanford ResearchSystems) was used for sampling and digitizing the signals. Forcontrolling the experiment; a homemade labview programwas employed. The data were fitted with the Fluofit program(Picoquant Berlin) employing an artificially created gaussianprofile deconvolution function to take into account the finitetemporal width of the excitation pulses. Its width was setto 120 fs FWHM, which was derived from the experimentalautocorrelation function of the Ti : Sa amplifier output.

The pump and probe beams were made to intersect at anangle of approximately 15 degrees, and the liquid solutionswere placed in a 1 mm glass cuvette centred on the crossingof the pump and probe beams.

The DASPMI dye was purchased from Molecular Probes(Invitrogen, SA), the Glycerol was from Riedel-de Haen, andboth were used without further purification.

The sol-gel media were produced as previously described[25], based on the method presented by Flora and Brennan[26], in the form of a monolith. Briefly, the sol was madeby mixing 9 mL of tetraethyl orthosilicate (TEOS) (Aldrich)with 3 mL of acidic water. This mixture was sonicated for1 hour and stored at −18◦C for about a month. The matriceswere produced by taking 2 mL of the sol and mixing it with2 mL of phosphate buffer solution (pH 7) containing thefluorophore DASPMI. The samples, prepared between twoglass plates, were about 1-2 mm thick. The outer edges ofthe glass plates were sealed with epoxy to prevent solventevaporation from the freshly prepared gel. The doped sol-gelmatrices were used within hours of preparation. Because ofphotobleaching of the dye, a new probing position was usedfor each time delay point. Since the samples contained smallspatial inhomogeneities, this caused a noticeable increase inthe noise level of the sol-gel measurements as compared withthose carried out in solution.

3. Results and Discussion

The dynamics of DASPMI kinetics were probed by stimu-lated emission spectroscopy in various glycerol : water mix-tures, in pure water, and in matrices produced via the sol-gel process for probe wavelengths of 560 nm to 620 nm. Therecovered parameters are shown in Table 1 and a majority ofthe decays are multiexponential in nature.

To exemplify, an illustration of a selected set of decaysis given in Figure 1. It shows the time-resolved stimulatedemission kinetics of DASPMI in different media at a probewavelength of 560 nm. The dramatic shortening of the emis-sion decay time upon lowering the viscosity is immediatelyobvious.

The strong dependence of the stimulated emission decayon the probe wavelength can be seen in Figure 2 for thelowest viscosity solvent (water). In addition to a shorteningof the decay time by about a factor of 100, we also observea rise time component for probe wavelengths of 600 nmand 620 nm. Tentatively this may also be present at 580 nm,although it was not possible to obtain a satisfactory fit fora temporally increasing component of the data available atthis wavelength. This rise time component may be attributedto a fast excited state relaxation of excited Frank-Condonstates, for example, due to a twisted geometry [11], and/ordue to solvent relaxation in the excited state [27] particularlyas a response to its CT-character. This view is supported bythe dramatic increase of the rise time when going from low-viscosity water solution to higher-viscosity glycerol mixtures(Table 1). Interestingly, we could not observe such a risetime in the sol-gel matrixes, which may be attributed to theabsence of this motional degree of freedom, although it is

International Journal of Spectroscopy 3

0 100 200

0

0.2

0.4

0.6

0.8

1

H2OGly 40%

Gly 80%Sol-gel

Time (ps)

Nor

mal

ized

sti

mu

late

d em

issi

on (

a.u

.)

50 150 250

Figure 1: Stimulated emission decay curves of DASPMI in differentmatrices at a probe wavelength of 560 nm.

0 10 20 300

0.2

0.4

0.6

0.8

1

0

0.5

1

Time (ps)

560 nm580 nm

600 nm620 nm

Del

ta O

D n

orm

aliz

ed

Time (ps)

5 15 25

0 1 2 3 4

Figure 2: Stimulated emission decay curves of DASPMI in water atdifferent probe wavelengths.

also possible that this component evades observation due tothe lower signal to noise ratio.

Table 1 summarises all experimentally determined stim-ulated emission decay fit values for water, two differentwater : glycerol mixtures (40% and 80% glycerol, expressedin mass percent) and the sol-gel system. Empty fields indicatethe absence of a decay constant in that time range. The timeranges are arbitrarily chosen and do not necessarily representthe presence or absence of a particular molecular process.It should be borne in mind that, in highly viscous media,inhomogeneous broadening of the kinetic components canoccur, leading to dispersive kinetics. Such processes are notexplicitly considered here but may contribute to the complexkinetics observed. The timescale of the parameters presentedin this table also shows that the use of an ultrafast systemallows an increase in the time resolution for emission studies,

as well as the possibility for exploring other excited statekinetic processes.

The values shown in Table 1 emphasise that studies withhigher time resolution compared to fluorescence decay aremore sensitive to probe local viscosity alterations in low-viscosity environments. It should also be noted that, ata viscosity of 45.9 cp [28] represented by a 80% Glycerolsample, the data approximately match those obtained withprevious fluorescence decay measurements [10]. We note,however, that the fastest decay component is shorter in thestimulated emission experiments.

Following the assessment of Strehmel et al. [11], basedon fluorescence decay data, the dynamics of DASPMI can beattributed to several processes: the direct transition from thefirst excited state to the ground state, a relaxation probablydue to a planar charge transfer state, and a slow relaxationrelated to a twisted charge transfer state.

It can be seen from these experiments that in solutionthe decay time of the shortest lived component, whichis attributed to the planar state of DASPMI, increasessignificantly upon augmenting the viscosity up to 3.1 cp.Further increases in viscosity by more than an order ofmagnitude only change the decay for a factor of two. Incontrast, a similar proportional increase in the decay timein the longer lived (here τ3) component attributed to aplanar CT state is seen at higher viscosities (on passing from3.18 to 45.9 cP). In the time-resolved stimulated emissionof DASPMI doped into sol-gel-derived media, only a singlecomponent is observed. However, in these studies, we couldnot follow the ultrafast decay kinetics with high precisionbut instead were limited to the intermediate time regime.This was because, in order to avoid photobleaching of thesample, different probe regions were selected. This led toincreased point-to-point fluctuations due to macroscopicsample inhomogeneities. Furthermore, the number of laserpulses that could be used to probe the solid medium waslimited by photobleaching, which, in turn, reduced themeasurement statistics.

In previous time-resolved fluorescence measurements,the decay kinetics has been reported to be multiexpo-nential, with three different lifetime components [10, 29].In comparison with those studies, it is possible that thelifetime component found here corresponds to the fastkinetic component in those experiments, but the dynamicsof DASPMI in the confined environment may also be dueto processes not observed in the homogeneous liquid phase.An indication for this is that the time constant for the directtransition from S1 to S0 is only weakly dependent on theviscosity, as witnessed by the small increase observed whengoing from 3.1 cp to 46 cp. In the sol-gel media, however, thetime constant is between 84 ps and 186 ps, much greater thanthe less than 20 ps observed in a solution with a viscosity of46 cp. In fact, a strong increase of the lifetime of DASPMIupon inclusion into sol-gel matrices has been observedbefore [13], and our data give strong evidence that DASPMIis not localized in microcavities of low viscosity solvent. Thisis particularly noticeable, as the hydrogels used in this studywere still in their initial stages of aging, at which the poresnetwork is still forming.

4 International Journal of Spectroscopy

Table 1: Stimulated emission decay times and relative amplitudes for various different water : glycerol mixtures and for the sol-gel matrix atthe different probe wavelengths. A negative amplitude indicates a rising rather than a decaying component. The sol-gel data could be fitted toa single exponential function with a residual amount around 25–30% that did not decay in the time window of the experiment. The viscosityvalues are taken from [28].

Sample Probe λ (nm) τ1 (ps) α1 τ2 (ps) α2 τ3 (ps) α3

Water (0.89 cp)

560 0.53 1580 0.67 0.59 9.86 0.41600 0.25 −1 0.57 0.54 8.46 0.46620 0.2 −1 12.1 1

Glycerol 40% (3.18 cp)

560 7.91 1580 1.7 0.59 34.3 0.41600 6.0 −1 6.1 0.41 69.9 0.59620 5.1 −1 8.3 0.60 38.5 0.40

Glycerol 80% (45.9 cp)

560 14.2 1580 18.9 0.12 255.7 0.88600 203.2 1620 4.73 −1 254 1

Sol-gel

560 84 0.73580 91 0.74600 128 0.75620 186 0.70

Also, note that the longer lifetimes are not seen in thesestimulated emission experiments, as the time range waslimited due to the length of the optical delay available forthese studies. However, the presence of longer excitationdynamics components can be deduced from a residualamplitude of around 25 to 30% representing excited stateswhich have not decayed within the time window of theexperiment.

In the solutions of different viscosity, the deviation in theshorter lifetimes, as compared to those measured by previousfluorescence lifetime measurements [10] could be due toa lower time resolution in time correlated single photon-counting experiments. In the previous measurements, theobserved lifetime is derived after deconvolution of a muchlonger instrument response function. Alternatively the tran-sitions observed in stimulated emission may be different onescompared to those in fluorescence decay. While fluorescenceis dominated by the decay of the relaxed excited state,stimulated emission processes can probe the molecule asit evolves from the initial excitation to the relaxed state,thereby providing complementary information on the probeenvironment.

4. Conclusion

Time-dependent stimulated emission was employed todemonstrate the potential of using DASPMI in matricesto characterise local viscosities, both in solution and sol-gel-derived media. It can be shown that time-resolvedstimulated emission experiments are promising for probinglocal environments in microheterogeneous environments,like polymers and other amorphous systems and biologicalsamples. The sensitivity of the effect can be compared tothat of the corresponding method of employing fluorescencelifetime spectroscopy techniques. However, the time scalefor stimulated emission experiments is much faster, as it is

basically limited by the pulse width of the laser employed,which in our case was around 100 fs. Consequently muchfaster processes can be accurately probed. In DASPMI, thetime evolution of the excited state scales with the viscosityof the solvent and an increased temporal resolution allowsone to greatly increase the range over which the localviscosity can be probed. Therefore, time-resolved stimulatedemission spectroscopy could be a suitable complementarymethod to time-resolved fluorescence techniques by whichto investigate lower viscosity samples.

It is known from previous work on sol-gel systemsthat there are kinetic processes on longer time scales asaccessible in the present stimulated emission setup. In futureexperiments, it is crucial to identify the physical processesassociated with the time constants observed in sol-gel matri-ces. It is not unreasonable to associate the subnanoseconddecay in sol-gel material with the faster reconfigurationprocesses in lower viscosity solvents as observed in the stim-ulated emission experiments presented here and in previousstudies using fluorescence upconversion [23, 27]. This wouldprovide additional insight into the interaction of DASPMI inthe sol-gel-derived microheterogeneous environment.

Acknowledgments

The authors acknowledge financial support from Fundacaopara a Ciencia e a Tecnologia (FCT) for the reequipmentGrant REEQ/25/FIS/2005. A. Rei was supported from FCTthrough the Ph.D. Grant SFRH/BD/27933/2006.

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International Journal of Spectroscopy 5

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